Separation of hydrogen gas from mixed gas streams is useful in a variety of systems aimed at chemical purification, waste stream management, and energy production, storage and use. One example of hydrogen separation technology is pressure swing adsorption (PSA). One way of implementing PSA processes is by use of a blower that draws gas into an adsorbent vessel. The adsorbent vessel separates gas constituents by their affinity for the adsorbent. The system regenerates spent adsorbent by reducing the pressure in the adsorbent vessel to remove bound gasses. Adsorbent regeneration may be facilitated by thermal effects. The concept of separation by adsorption affinity is not limited to hydrogen gas and may include, for example, separation of carbon monoxide or other impurities.
Pressure related systems in the nature of PSA process are relatively expensive to install and maintain. Hydrogen removal by these devices may be thermodynamically inefficient and relatively expensive. Inefficiency of separation may also occur in cases where components of mixed gas flow streams have similar affinities for the adsorbent.
Another hydrogen separation technology involves the use of hydrogen selective membranes, which are generally made of one of three material classes: organic polymers, metals, and ceramics. In membrane separation, the hydrogen transport rate is proportional to the hydrogen partial pressure differential across the membrane and the hydrogen diffusion across the membrane. The hydrogen transport rate is inversely proportional to the thickness of the membrane. Hydrogen transport is also a function of temperature, and increases with increasing temperatures. For some materials that cannot withstand harsh environments of use, it is problematic that most large scale, industrial processes for the production of hydrogen operate at high temperatures, e.g., from 700° C. to 1200° C., and high pressures ranging from several hundred psi to 1000 psi.
Polymer membranes achieve separation via selective dissolution within the polymer of certain species within a mixed gas stream. The dissolved species are then transported across the membrane. A major drawback of these membranes is their limited thermal stability and poor mechanical strength, which renders them unsuitable at prevalent commercial operating conditions.
Metallic separation membranes are based on catalysis, for example, with Pd alloys. These materials transport hydrogen in atomic form. Hydrogen dissolves in the structure and diffuses across the membrane from one side to another under bias of partial pressure. After selective transport, the hydrogen emerges as molecular hydrogen, H2. Although hydrogen transport is rapid in these systems, the alloys are expensive. The alloys may lack suitable thermal stability and/or mechanical properties for commercial operation at conventional high temperatures and pressures.
Ceramic membranes can be fabricated from thermally stable and mechanically strong inorganic materials and are thus more viable than polymer and metallic membranes at commercial operating conditions. These membranes may be divided into two general categories based on their transport mechanisms, namely, permselective transport and ion conducting transport.
Permselective membranes are fabricated with porosity on a nanometer scale. The porosity is sized or scaled to allow small molecules such as hydrogen to pass. The scale impedes passage of larger molecules. Surface chemistry processes, such as surface adsorption, may affect transport rates and aid in obtaining higher selectivities. One disadvantage of permselective membranes is that they are difficult to fabricate with control and repeatability over large surface areas. A very fine control of porosity is required to achieve high selectivity for hydrogen over other components in a mixed gas stream.
Ion conducting ceramic membranes (“ICCMs”) operate by selectively transporting protons, i.e. hydrogen ions, across a membrane. The protons are formed by electrochemical reactions at a membrane surface for transport across the membrane under bias of partial pressure. The transport mechanism does not necessarily utilize nanostructured porosity, and the transport rate is affected by temporary electrostatic or bonding interactions as the protons pass through the ceramic lattice. Emerging hydrogen ions recombine with electrons to form molecular hydrogen, H2. Selectivity for hydrogen transport may approximate 100% when the transport mechanism is based on proton conductivity and the membrane is not physically porous. Since ICCMs are fabricated from thermally stable and mechanically strong ceramics, they are compatible with the prevalent temperatures and pressures of commercial processing. The ceramics used in ICCMs are usually solid metal oxides. The crystal structure is frequently that of a perovskite or a pyrochlore.
Materials selection and design may require tradeoffs between various factors affecting the net rate of hydrogen transport across an ICCM hydrogen separation membrane, such as: (1) the area of the membrane, (2) the thickness of the membrane, (3) the rate of the electrochemical reactions at the input gas side, (4) the concentration of protons and electrons that the membrane is capable of holding in a dissociated state, (5) the mobility of the protons and electrons within the membrane, (6) the rate of the electrochemical reactions at the output gas side, and (7) the differential partial pressure of hydrogen gas across the membrane.
One such tradeoff is illustrated, for example, by using a larger area to increase hydrogen transport. The larger surface area undesirably increases the overall size and weight of the hydrogen separation system. In another example, using a thinner membrane may increase transport rates, but thinner membranes are vulnerable to hole formation in manufacture, as well as pressure ruptures in the commercial environment of use.
Hydrogen throughput of ICCMs may often be increased by raising the hydrogen partial pressure differential across the membrane. This raising of hydrogen partial pressure differential may be accomplished by (1) maintaining a high pressure of input gas and a low pressure of output gas, and/or (2) flushing the output gas surface with a carrier gas so that molecular hydrogen is promptly removed, lowering the partial pressure of hydrogen gas at the output side. With these available process controls, the remaining factors to improve proton throughput are materials related factors, such as concentrations and mobilities of protons and electrons within the membrane.
One class of ICCMs includes rare-earth-doped alkaline earth cerates, which are exemplified by gadolinium-doped barium cerate (BaCe0.9Gd0.1O2.95) and yttrium-doped strontium cerate (SrCe0.9Y0.1O2.95). These materials can be used to obtain high proton conductivities, but disadvantageously react with carbon dioxide to form barium or strontium carbonates, as the case may be. For the most part, industrial scale hydrogen production processes produce a mixed gas stream that contains carbon dioxide, and consequently, this class of ICCM suffers physical degradation by reacting with carbon dioxide in the intended environment of use.
A second class of ICCMs described in current literature is based on doped lanthanum zirconate, La2Zr2O7. In yttrium-doped lanthanum zirconate, proton conductivity is enhanced relative to undoped lanthanum zirconate, but is unsuitably low for industrial applications. Calcium-doped lanthanum zirconate shows even lower proton conductivity. Structural problems may arise from the use of calcium-doped materials, such as formation of carbonates in the presence of carbon dioxide, as described above. Calcium materials may also be associated with formation of undesired phases, such as calcium zirconates.
U.S. Pat. No. 5,403,461 issued to Tuller et al (“Tuller”) describes solid solutions that demonstrate ionic conductivity. Although specific element substitutions for enhancing ionic conductivity are disclosed, Tuller does not elaborate on the mechanism by which the disclosed substitutions enhance ionic conductivity. Also, the compounds and experimental investigations discussed in Tuller are directed exclusively to oxygen ion conductivity, rather than proton conductivity. Finally, the elemental formulae of Tuller are drawn so broadly as to cover an enormous spectrum of compounds, some of which have no known or useful ionic conductivity. For instance, equal utility under the same formula may be asserted for diverse compounds including lead oxide Pb3O4; lead iron tungstate PbFe0.67W0.33O3 (a dielectric); yttrium aluminum garnet Y3Al5O12 optionally doped with Nd or Ho, and yttrium orthovanadate YVO4 (both laser materials); material yttrium iron garnet Y3Fe5O12 (a ferromagnetic); YBaCuO7, (a superconductor); and strontium barium niobate SrBaNb4O12 (a transparent ferroelectric).
As used herein, the terms “lanthanide elements” or “lanthanides” shall refer to elements including La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.